ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Friday December 28th, 2018
Posted by Heather Thamm on 12/28/18 at 7:00 am.
Avalanche risk
The Bottom Line
Moderate Avalanche Danger
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

The avalanche danger is MODERATE above 1000′ for triggering an unstable cornice or a new/old wind slab.   Triggering a slab on an older layer 2+’ deep is becoming less likely, but not impossible in the periphery of the forecast zone near Johnson Pass and Silvertip.   Identify glide cracks and avoid traveling under these unpredictable avalanche hazards.

JOHNSON PASS / LYNX DRAINAGE / SUMMIT LAKE:    South of Turnagain Pass weak layers exists under 2-3′ of snow. Human triggered slab avalanches over 2′ thick are possible on slopes over 35 degrees.

LOST LAKE:    This zone is out of the advisory area, but is also suspect for harboring weak layers 2-3′ below the snow surface due to recent reports. Pay attention for signs of instability like collapsing and recent avalanches.  

2. Moderate
Alpine
/ Above 2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
2. Moderate
Treeline
/ 1,000'-2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
1. Low
Below Treeline
/ Below 1,000'
Avalanche risk
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

Special Announcements

For the Hatcher Pass Avalanche Center mid-week summary click  HERE.

Avalanche Problem 1
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
  • TYPE
    Cornice
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

Cornices remain large and looming along many ridgelines of our forecast zone. On Sunday and Monday evidence of natural cornice fall was observed and several human triggered cornices were reported. The best way to manage this hazard is to give cornices a lot of space and avoid travel directly underneath them. Approach ridgelines with caution and be aware of groups traveling in the same zone as you who could inadvertently triggered one from above.

WIND SLAB: There is plenty of snow available for transport and we may see winds pick up into the 20’s mph later in the day. There is a layer of buried surface hoar below 2-5” of new snow from the last few days and this may become our next weak layer. Today keep an eye out for drifting snow and shooting cracks. These are signs of fresh wind slab development. Triggering an older wind slab is becoming pretty unlikely at this point, but not out of the question in very steep terrain. A period of strong winds on Christmas has created some stiffer pockets of snow in the Alpine. 

Large cornices still intact on the SW facing terrain of Magnum. This photo also has some natural cornice fall observed on Mon (12.24) as well as glide cracks opening up in the lower right corner. 

 

A thin layer of surface hoar lays below a few inches of snow accross our forecast zone and will be a suspected weak layer if winds pick up and form wind slabs. 

Avalanche Problem 2
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. The are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
  • TYPE
    Glide Avalanches
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

The most recent glide avalanches observed were in Lynx Creek drainage on Monday. Numerous glide cracks have been slowly opening up around our region including commonly traveled areas like the SW aspect of Sunburst and Corn Biscuit, and in Warm-up (-1) Bowl on Seattle Ridge. It is important to remember glide cracks can release into full-blown avalanches at any time and are not associated with human triggers. The best way to manage this problem is to keep your eyes peeled for cracks and limit travel underneath them.

Glide crack in Warmup (-1 Bowl) on the North facing slope. This is a common crack that forms and has been known to release. Photo taken on 12.24 by Wendy Wagner.

 

Additional Concern
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
  • TYPE
    Persistent Slabs

If you are headed to areas south of Turnagain, keep in mind triggering a large slab avalanche is still possible. Buried weak layers, roughly 2-3′ below the snow surface, have been found in the Summit Lake zone and as far south as Lost Lake. Numerous observations this week in Turnagain Pass have been finding a stable snowpack in the older November snow near the ground. Some uncertainty remains around Johnson Pass, Lynx Cr drainage and Twin Peaks/Silver Tip where the snowpack often transitions to a shallower structure similar to Summit Lake. Observations in Summit Lake over the last few weeks have been finding facets associated with several crusts near the ground with reactivity in stability tests. The most recent avalanche activity in this zone was observed last week (Dec.20) where some of these avalanches stepped down to older layers within the snowpack. Should you head into the periphery of our forecast zone evaluate the snowpack and terrain as you travel, and keep your senses alert for any whumpfing — an obvious clue for triggering a slab avalanche.

Mountain Weather

Yesterday: Skies were obscured and snow showers left a few inches of new snow (1-3 €) across our region. Temperatures decreased into the low 30Fs near sea level and mid 20Fs at 1000′. Ridgetop winds remained light from the East shifting to the North overnight as skies cleared.

Today: Expect partly cloudy skies in the morning becoming mostly cloudy by the afternoon. Snow showers are expected by late afternoon and into the evening with 3-7 € possible overnight. Ridgetop winds should start out calm and could increase to 10-25mph from the East by the afternoon. Temperatures should be similar to yesterday and remain cool enough for snow at sea level.

Tomorrow: Snow flurries will taper off in the morning with another break between snow showers. We may see another period of partly cloudy skies before another storm moves into our region on Sunday through Monday. There remains some uncertainty around how much precipitation, but expect a period of warmer temperatures and increased winds.

*Seattle Ridge weather station was heavily rimed and the anemometer (wind sensor) was destroyed.    

PRECIPITATION 24-hour data (6am – 6am)

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 27   1   .1   55  
Summit Lake (1400′) 26   1   .1   13  
Alyeska Mid (1700′) 26   3   .27   37  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20   ENE   4   19  
Seattle Ridge (2400′) 25   *N/A   *N/A     *N/A